The long range objective of the proposed research is to contribute to extending and enhancing diagnostic ultrasound by elucidating the physical principles underlying the use of nonlinear imaging. Echocardiography evolved from linear (fundamental) imaging to nonlinear (harmonic) imaging in a remarkably short period of time. The instrumental advances that facilitated this rapid evolution were initially developed to capitalize on the properties of contrast agents. However, it soon became clear that the nonlinear properties of tissue alone were sufficiently strong that images could be formed at frequencies near 2-f from an ultrasonic beam transmitted at 1-f. For many patients, the resulting harmonic images are of notably higher quality than the corresponding fundamental images. The rationale that underpins our proposed research is that a better understanding of the physics responsible for the generation (with distance traveled) and the propagation of the nonlinear signals, which arise from the (linearly generated) fundamental ultrasonic field developed at the face of the imaging transducer, will lay the foundation for even more significant improvements in image quality. We propose experimental studies under laboratory conditions of nonlinear ultrasonic propagation in excised hearts and other tissues interrogated through phase-aberrating media analogous to chest or abdominal wall, and in tissue-mimicking media with well-controlled attenuation, backscatter, and speed of sound. We propose to compare the results of our measurements with predictions based on a Burgers equation enhanced, nonlinear angular spectrum simulation approach. Using the results of these experiments and simulations, we propose to evaluate the strengths and limitations of the concept of "effective apodization" to characterize the central features of the diffracting and attenuating nonlinearly generated field as it propagates in the patient. The proposed research is designed to enhance the role of ultrasound in the clinical environment by providing definitive answers to a detailed series of explicitly posed questions.